Generate a proper version string when using git archive

this enable github-generated tarballs to work correctly

CMakeLists.txt

@@ -97,17 +97,17 @@ if(NOT SOURCE_PACKAGE)
   EXECUTE_PROCESS(COMMAND git describe --tags HEAD #OUTPUT_VARIABLE PROJECT_VERSION OUTPUT_STRIP_TRAILING_WHITESPACE
       COMMAND sed "s,^release-,,;s,-,+,;s,-,~,;" OUTPUT_STRIP_TRAILING_WHITESPACE OUTPUT_VARIABLE PROJECT_VERSION
       WORKING_DIRECTORY ${CMAKE_SOURCE_DIR})
-
-  # and here we write out the version into a .h file suitable to be used in the code.
-  # the file only gets written when the version is actually different to avoid unneeded recompiles
-  add_custom_target(
-    generate_version ALL
-    COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/tools/create_version_h.sh ${CMAKE_CURRENT_BINARY_DIR}/src/version_gen.h
-    COMMENT "Updating version string"
-    WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
-  )
 endif(NOT SOURCE_PACKAGE)
 
+# and here we write out the version into a .h file suitable to be used in the code.
+# the file only gets written when the version is actually different to avoid unneeded recompiles
+add_custom_target(
+  generate_version ALL
+  COMMAND ${CMAKE_CURRENT_SOURCE_DIR}/tools/create_version_h.sh ${CMAKE_CURRENT_BINARY_DIR}/src/version_gen.h
+  COMMENT "Updating version string"
+  WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
+)
+
 #
 # Initial cmake/debian/postinst and prerm script for debian package
 #

tools/.gitattributes

@@ -0,0 +1 @@
+create_version_h.sh export-subst

tools/create_version_h.sh

@@ -6,7 +6,12 @@ H_FILE=$1
 
 VERSION_H_NEEDS_UPDATE=1
 
-NEW_VERSION=`git describe --tags HEAD | sed 's,^release-,,;s,-,+,;s,-,~,;'`
+NEW_VERSION=`git describe --tags --dirty | sed 's,^release-,,;s,-,+,;s,-,~,;'`
+
+# if we are not in a git checkout, NEW_VERSION is empty
+if [ -z "${NEW_VERSION}" ]; then
+	NEW_VERSION="archive-$Format:%H$"
+fi
 
 # version.h exists => check if it containts the up-to-date version
 if [ -f ${H_FILE} ]; then